It is well known that shaft furnaces are ideally suited for the direct reduction of oxidic ores notably iron ore, due to their ability to pass the ore to be reduced in countercurrent relation to the reducing gases thereby to improve the thermal efficiency of the process as well as to enhance reactant effectiveness. In these furnaces iron oxide pellets, course (lump) ore, or other iron-bearing agglomerates are reduced to iron in the solid state by the removal of as much as 95 percent or more of the contained oxygen by passing high temperature reducing gas, at about 1600.degree. F. or higher, upwardly through the furnace counter to the downwardly descending pellets. The reducing gas is rich in both hydrogen and carbon monoxide which are each thermodynamically favorable to the reduction of iron oxide.
In U.S. pat. appln. Ser. No. 644,249, filed by S. Waslo and assigned to the assignee herein; there is discribed a method of operating a shaft furnace of the aforementioned type in a manner to derive an improved product that is less prone to degradation and to reoxidation. In the method described in this patent application only part of the reducing gas is admitted to the shaft furnace at an elevated temperature. The remainder is introduced at about ambient temperature and in controlled amounts to maintain the temperature of the ore in the cooling section of the furnace above a prescribed minimum temperature and for an extended period of time.
In order to obtain optimum performance in the described process, it is necessary that only so much low temperature reducing gas be admitted to the furnace chamber as is necessary for cooling the reduced product. In cases where an excessive amount of cold gas is introduced to the furnace the heating and reduction of the ore particles is adversely affected. Should an insufficient amount of cold gas be supplied the product emerging from the furnace chamber will be at undesirably high temperatures rendering the product prone to reoxidize.
In practice, therefore, it is desirable to match the heat capacity of the gas flow with the heat capacity of the reduced product to control the admission of cold reducing gas. The effective matching of these flows is difficult, if not impossible to obtain without the provision of substantially uniform gas flows across the transverse section of the furnace chamber. This latter condition is, itself, difficult to achieve due to the fact that, because the emission of high temperature gas to the heating and reducing section of the furnace is effected through tuyeres disposed in the shell wall, the gases are not immediately distributed across the furnace section the heating of the particles and reduction process attendant therewith is nonuniform across the furnace section. The problem of maldistribution of reduction gases is compounded in the lower, cooling section of the furnace by the fact that, due to viscosity considerations, the cold reducing gas admitted to the furnace chamber is attracted to the lower temperature regions thereof such that the hotter regions remain hotter. Maldistribution of reduction gases is further aggravated in this region of the furnace by reason of the fact that because solid particle flow varies from location-to-location transversely of the section there is created a concomitant variation in gas flow through the respective locations.